THE TYPE PI CARRIER SYSTEM 



355 



3.2 Use of Transistors 



Transistors were chosen for use in the PI system because they are low 

 voltage, low power devices as compared to electron tubes suitable for 

 transmission circuitry. Also, transistors are expected to be lower in cost 

 and inherently longer life devices than electron tubes, thus contributing 

 to reduced initial and operating costs. 



The dc power requirements for the PI system, using transistors, may 

 be compared to those for a channel terminal in the Type Nl system as 

 an indication of the dc power saving that has been achieved with the 

 Pi system. A transistorized PI terminal requires about 1.2 watts while 

 it is in operation compared to 40 watts required for an Nl terminal, 



INPUT 



OUTPUT 



' ■ BATTERY 



Fig. -3. — PI transistor transmitting amplifier circuit. 



which represents a substantial power reduction achieved by the use of 

 transistors. Because part of the PI terminal is turned off during idle 

 periods, the average power required over a day is about 0.9 watt. 



During the development of the PI terminals it was found that a single 

 design of a transistor amplifier could be used in se^'eral different places. 

 These included the compressor and expandor amplifiers, the transmitting 

 amplifier and the input portion of the receiving amplifier. The circuit 

 for that amplifier is shown in Fig. 3. 



The amplifier uses Western Electric NPN grown junction type tran- 

 sistors coded 4B for the voice frequency amplifiers and 4C transistors for 

 the carrier amplifiers. The first transistor is connected as a common col- 

 lector and the second as a common emitter. By using them in this man- 

 ner it is possible to employ the same type of transistor in both stages. 

 Feedback is obtained by using hybrid coils at both the input and output 



